Handheld printing devices are commonly employed for printing indicia such as price tags, electrical wire markers, and the like. Such devices print indicia on various types of available media. Regardless of the type of media employed, it is necessary that the printed indicia be clearly marked on the media so that the information thereon may be clearly conveyed. The typical printing devices include printing heads that may be either manually or electronically set to print the desired indicia on the media.
Printing devices must provide labels that are legibly printed. Of significant concern is that the printing device provide printed labels in a consistent and predictable manner to the printing plane of the printing head. In that regard the printer must steadily support the printing surface of the media in the printing plane so that the marking applied by the printing head thereon is legible. In order to support the media during printing, a platen of some type is employed. The platen typically includes either a portion of a flat plate or a rolling cylinder against which a print head strikes to print indicia on a label held against the platen.
It is well known that in printing labels for wire marking purposes it may be desirable to print the indicia on various types of media. Such media may include paper or film labels or, in the case of wire markers, polymer tubes. Thus the media thickness will vary, depending on the type of label and the application. The position of the printing surface of the media must be precisely maintained in the printing plane as the printing head applies ink to the media. The thickness of the label can affect the printing performance since, for a given printing plane, the relative position between the platen and the printing surface of the print media will vary with the thickness of the media. For example, in order to precisely locate the printing surface of a print media with respect to the printing plane of a printing head, thicker print media requires that the platen be located further from the printing plane and thinner print media requires that the platen be located closer to the printing plane. The platen and the printing head may therefore need to be relatively adjustable so as to accommodate print media of varying thicknesses.
Relative movement between the platen and the printing head is shown in the prior art labeling devices.
U.S. Pat. No. 3,463,084, to Nineberg, shows a planar platen on which print media is stacked in a staggered fashion thereon. The exposed margins of each layer of stacked media passes beneath a rotary printing cylinder. The platen is mounted on springs so that as each successive layer of the stacked media comes under the rotary cylinder, the cylinder drives the stack, and thereby the platen, downward so that the printing plate of the rotary cylinder engages each exposed margin. The platen of the '084 patent is employed in a large non-portable machine.
U.S. Pat. No. 4,144,810, to Sato, shows a planar platen as part of a handheld printing machine with opposing spring assemblies for urging a print media on the platen to briefly engage a printing device. The platen is continuously biased against the printing device and a restraining means blocks motion of the platen toward confrontation with the printing device. Not until the operator squeezes a hand lever will the platen be free to momentarily urge the print media into the printing plane of the printing device. The '810 patent, however, does not provide for accommodating print media of varying thicknesses in the printing plane of the printing head.
U.S. Pat. No. 4,206,703, to Sato, shows a handheld label printing machine with a pivotably movable planar platen that pivotally raises the print media to the printing head when hand levers are pressed together. When the operator relaxes the grip, the platen is pivotally lowered from the printing head. The '703 patent likewise fails to provide for accommodation of print media of varying thicknesses in the printing plane of the printing head.
U.S. Pat. No. 4,843,338, to Rasmussen, shows a plate-type platen which is biased by compression springs, so as to be urged farther away from the printer when thicker media is fed between the platen and the printhead. This platen assembly is held stationary at one end and is movable against a spring assembly at the other end. The platen thus pivots to accommodate thicker print media such that the platen is at an angle with the printing plane. Furthermore the platen is curved inflectivey to the curvature of the delivery mechanism so that the platen imparts a reversed bow in the print media with respect to the delivery mechanism. However, as the platen of this device pivots, it changes the alignment between the printing surface of the print media and the printing plane. Changing the thickness of the print media may result in a certain degree of misalignment. Furthermore, the '338 patent does not disclose a handheld printing device as it is directed to a large stationary printing device.
U.S. Pat. No. 5,372,443, to Borucki, shows a printer assembly with a pivotable platen having a curved portion at an end that urges the print media against a stationary mask. At the opposite end from the curved portion, a torsion spring wound around the pivoting axis of the platen urges the curved portion of the platen against the stationary mask. The guidepath delivering the print media to the platen-mask includes opposing rollers. The roller on the platen side of the print media is fixed while the opposing rollers are spring biased. The '443 patent provides another example of a platen that pivots and therefore is unable to maintain the alignment between the printing surface of the print media and the printing plane of the printing head for print media of various thicknesses. While the device shown in the '443 patent is designed to accept such media, the device includes complex mechanical components which rely on precise pivotal movement to adjust for variations in media thickness.
It is therefore desirable to provide a hand held printing device which will accommodate print media of varying thicknesses and maintain the alignment between the printing surface of the print media and the printing plane of the printing head and which is less complicated and more reliable than the prior art devices.